The secretion of stored granular components, known as the release reaction, is a central event in platelet function. The reaction occurs is response to a variety of stimuli and leads, under appropriate conditions, to the formation of platelet aggregates. Consequently, secretion by platelets contributes in a major way to hemostasis. Patients exhibiting hemostatic defects can be suffering from a variety of lesions in the sequence leading to the creation of a platelet plug. One such lesion can exist in the process of release. An approach for ascertaining the factors responsible for abnormal release involves studying the lipid metabolism of platelets exposed to secretion-inducing stimuli. Platelets, like a number of other secretory cells, display a striking shift in phosphoinositide (PI) metabolism when challenged with agonists such as thrombin, ADP, epinephrine, or collagen. The role played by PI in the secretory process has yet to be elucidated. However, perturbations in PI turnover are so consistently associated with stimulus-secretion that closer study of PI metabolism in platelets is warranted. Defects in PI metabolism may underlie certain abnormalities in human hemostasis and thrombosis. To dissect the various aspects of PI metabolism, we propose to examine 1) endogenous levels of PI-phosphodiesterase activity and 2) labeled glycerol incorporation into PI of platelets undergoing secretion in response to thrombin. In related studies, we intend to analyze 3) the amount of PI at the exterior of the platelet plasma membrane 4) the branch point enzymes controlling PI synthesis which display altered activity during secretion 5) the extent of synthesis of myo-inositol by human platelets 6) whether an association exists between PI and a platelet membrane protein component, and whether it is alterable by exposure of platelets to thrombin. In conjunction with this work, we expect to examine platelets displaying defective secretion from eight patients with recognized functional defects.